Electrolytic polishing of zirconium, hafnium and their alloys



Sept. 15, 1959 A. T. MCCORD ETAL 2,904,479

ELECTROLYTIC POLISHING 0F "'ZIRCONIUM, HAFNIUM AND THEIR ALLOYS Filed April 24, 1956 2 Sheets-Sheet 1 INVENTORS ANDREW 7: Mc CORD DONALD R. SP/NK A TTORNE Y Sept. 15, 1959 A. T. MQCORD ETAL 2,904,479

ELECTROLYTIC POLISHING OF ZIRCONIUM, HAFNIUM AND THEIR ALLOYS Filed April 24, 1956 2 Sheets-Sheet 2 TESTED IN WET CARBON DIOXIDE AT 500C.

g ELECTRO-POLISHED o 2O 4O 6O 80 I00 I20 TIME DAYS TESTED IN WET l2 CARBON DIOXIDE AT 500 C.

CORROSION RATE MILS/YEAR O) COMMERCIAL ZIRCONIUM l \I i COMMERCIAL ZIRCONIUM- ELECTRO-PQLISI iED 2o 40 so so I00 I20 TIME- DAYS JNVENTORS ANDREW T. Mc CORD DONALD R. SPINK ATTORNEY United States Patent ELECTROLYTIC POLISHING OF ZIRCONIUM, HAFNIUM AND THEIR ALLOYS Andrew T. McCord, Snyder, and Donald R. Spink, East Amherst, N.Y., assignors to The Carborundnm Company, Niagara Falls, N.Y., a corporation of Delaware Application April 24, 1956, Serial No. 580,311

7 Claims. (Cl. 204--140.5)

This invention relates to a process for the electrolytic treatment of zirconium, hafnium, and their alloys whereby the surface of the metal is converted to one of high luster through said treatment. Moreover the resulting polished metal articles are unexpectedly improved in resistance to corrosion.

It has heretofore been proposed to polish metals of Group IV-A of Mendeleeffs Periodic Table by immersing them in highly acid etching solutions such as hydrofluoric acid or mixtures of hydrofluoric and nitric acids or to the usual mineral acids including hydrochloric acid at various concentrations up to the boiling point. The acids at tack the metal and impart a semi-polished surface to the treated object. These acid solutions often unevenly attack the surfaces of the metal so as to bring out the grain structure, and more truly are referred to as etching solutions. Such solutions because of their highly corrosive character are very difficult to handle, and are sometimes violent in action and hard to control.

It is an object of the present invention to provide a new and improved method of polishing zirconium and hafnium and their alloys which obviates the use of highly corrosive acidic solutions and the problems and difliculties attendant thereto.

It is a further object to provide a method of treating zirconium, hafnium, and their alloys that Will not only impart a polish or luster to the treated metal surfaces but will impart to the treated surface an increased resistance to certain corrosive influences.

We have discovered the highly effective solvent action of neutral solutions of ammonium fluoride upon metals such as zirconium, hafnium, and their alloys. In the course of our experimental work involving the dissolution of zirconium, hafnium, and their alloys in ammonium fluoride solutions the effect of a direct current (D.C.) on the solubility of the metal in the bath, using the metal as both the anode and the cathode, was investigated. When this was attempted using the metal as the anode, there was a pronounced decrease in attack of the ammonium fluoride solution upon the metal and the metal attained a brilliant polished surface or luster. A further investigation led to the added finding that the resulting polished metal body had been markedly improved in resistance to corrosion under exposure to Wet carbon dioxide gas at 500 C. and to boiling hydrochloric acid. We have therefore provided a new and improved method of treating zirconium, hafnium, and their alloys by which the surface of such metals can be effectively polished and at the same time be substantially improved in resistance to corrosion. The process of treatment comprises immersing the metal object the surface of which is to be polished and/or rendered corrosion resistant in a substantially neutral solution of ammonium fluoride, making the metal the anode and the solution the electrolyte of an electrolytic cell circuit with suitable cathode provided, and passing an electrical current through the circuit at a sufiicient anodic current density and for a sufficient length of time to produce the desired surface on the metal anode.

Figure 1 is a diagrammatic View of an electrolytic cell circuit embodying the principles of the present invention and adapted for the carrying out of the present process. As shown, the cell chamber 3 of suitable size for carrying out the intended operation is filled with a bath 4 of concentrated ammonium fluoride solution. The metal object 5 which is to be polished is suitably connected to a piece of copper Wire or other metal wire of suitable resistance to attack in the electrolytic bath and is immersed in the solution to serve as the anode. Carbon electrodes 7 and 8 are likewise connected to lead Wire 9, and leads 6 and 9 connected to a suitable source of electrical power 12 to complete the circuit. Ammeter 10 and resistance rheostat 11 are included in the circuit as means to control the anodic current density of the bath.

'Figure 2 is a sketch similar to that in Figure 1 With like parts identified by similar reference numerals but differing therefrom in that the cell chamber 14 constitutes the cathode of the electrolytic cell circuit in place of the cathodes 7 and 8 of Figure 1.

Figure 3 is a graph showing the rate of corrosion in mils per year for Zircaloy-Z zirconium alloy in wet carbon dioxide at 500 C. over periods up to as high as days. The graph compares the rate of corrosion of the untreated alloy and also the rate of corrosion of the same curves for zirconium metal, both untreated and electropolished, in Wet carbon dioxide at 500 C., and again it is observed that the electropolished specimen has been substantially improved in resistance to corrosion over that of the untreated metal.

The present process of treating zirconium, hafnium, and their alloys is carried out at room temperature in a concentrated solution of ammonium fluoride, as for example a solution of 500 grams of ammonium fluoride per liter of Water, and having a pH of from 6.5 to 7.0. The metal object to be polished is secured to a suitable lead Wire of copper, zirconium, molybdenum or other metal which is suitably resistant to corrosion in the bath and then immersed in the ammonium fluoride solution as the anode of the electrolytic cell circuit. One or more pieces of carbon either in rod or slab form are then inserted in the ammonium fluoride solution and constitute a suitable cathodic means of the circuit to assure flO'W from all parts of the anode.

Inasmuch as the ammonium fluoride solution is severely corrosive to zirconium and hafnium metals when there is no implied voltage, the initial voltage is quickly applied so as to minimize metal losses in the bath and the applied voltage is maintained on the object being treated until it has been removed from the bath and thoroughly washed. Fairly low voltages are used with corresponding current densities in the range of from 1 to 4 amperes per square decimeter. Higher current densities result in shorter polishing times although if the current density is too high pitting, uneven polishing and even burning of the specimen occurs. Stirring of the ammonium fluoride solution has been found to be advantageous in the production of uniformity of treatment of the metal surface. Furthermore, using the same voltage, higher current densities are produced Without stirring than with stirring. The polishing of zirconium, hafnium, and their alloys in accordance with the present process is effective, only several minutes in the bath being required to produce an optimum surface not only with respect to the degree of luster or polish imparted to the surface but also 3 the development of the ability of the surface to resist corrosion. Metal losses in polishing have been determined for a number of different pieces and are found to average from 1 to 35 milligrams per square decimeter per hour.

It has been found that the polishingaction is absolutely uniform on all surfaces of the metal piece being treated and is in no Way dependent on the material, shape, size or location of the cathode with respect to the anode. For example, both zirconium and Zircaloy-2 zirconium alloy tubing have been polished on inside and outside surfaces in the same solution and with the same cathode that was used satisfactorily for the polishing of sheet stock. The inside surface of the tubing was identical in appearance to the outside surface. Also, any size or shape of metal 'to be polished in accordance with the herein-described method, and irrespective of the intricasies of contour it has been found that the polishing action extends to all surfaces of the piece to an ab solutely uniform degree.

It has also been found that the addition of small amounts of other metallic fluorides to the electrolytic bath improves the polish or luster produced using the methods herein-described. For example, the introduction of copper into the solution as an ammoniacal copper complex has appeared to increase the brillianceof the'surface'of treated metal articles.

While we have described the present process as one to be used in place of the conventional nitric-hydrofluoric acid polishing or etching procedures, the present process can also be used in conjunction with such conventional polishing or etching treatments as a subsequent polishing operation subsequent to the conventional acid etch to obtain the ultimate in surface brilliance or other special effect as desired.

Having described the invention in detail it is desired to claim:

1. A process of electrolytically polishing a metal selected from the group consisting of zirconium, hafnium, and their alloys comprising making the metal the anode 4 in an aqueous, substantially neutral solution of ammonium fluoride and passing a direct electric current therethrough of sufficient currentdensity and for a sufficient length of time to produce a polish on said metal.

2. A process of anodically polishing a metal selected rom the group consisting of zirconium, hafnium, and their alloys comprising immersing the metal in a concentrated solution of ammonium fluoride as the electrolyte, making said metal the anode in an electrolytic circuit including said electrolyte, and passing a direct electric current through said circuit at a su'fficient anodic current density and for a sufficient length of time to effect polishing of the metal.

3. A process according to claim 2 in which an ammoniacal copper complex is added to the electrolytic solution.

4. A process of electrolytically polishing a metal selected from the group consisting of zirconium, hafnium and their alloys comprising making the metal an anode in an ammonium fluoride solution as the electrolyte, providing a suitable cathode to assure uniform flow from all parts of the metal anode, and passing a direct electrical current through the resulting cell at a low voltage and a current density of 1 to 4 'arnperes per square de'cimeter of'anodic surface.

5. A process according to claim 2 in which the voltage of the electrical-current is at 2 to 10 volts an'd'th'e anodic current density is 1 to 4 amperes per square decimeter of anodic surface.

6. A process according to claim 2 in which the ammonium fluoride electrolyte .is maintained in circulation.

7. A process according to claim 2 in which the ammonium fluoride solution has a concentration of 200 to 550 grams of ammonium fluoride per liter of solution.

References Cited in the file of this patent FOREIGN PATENTS 294,237 Great Britain Sept. 12, 1929 i "War: V 1 

2. A PROCESS OF ANODICALLY POLISHING A METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, HAFNIUM, AND THEIR ALLOYS COMPRISING IMMERSING THE METAL IN A CONCENTRATED SOLUTION OF AMMONIUM FLUORIDE AS THE ELECTROLYTE, MAKING SAID METAL THE ANODE IN AN ELECTROLYTIC CIRCUIT INCLUDING SAID ELECTROLYTE, AND PASSING A DIRECT ELECTRIC CURRENT THROUGH SAID CIRCUIT AT A SUFFICIENT ANODIC CURRENT DENSITY AND FOR SUFFICIENT LENGTH OF TIME TO EFFECT POLISHING OF THE METAL. 